Fix error message for some failed operations

[DrRataplan]

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I just reran the QT3 tests with npm run qt3tests -- --regenerate. My output contained a lot of changes like this:

-extvardef-007,Error: Not implemented: adding durations to xs:date
+extvardef-007,Error: Not implemented: adding durations to 7

This is caused by the #342 PR. There should be a stringification happening where that error is generated. Plus a manual unit test.